1. Field of the Invention
This invention relates to an automatic track feeding (hereinafter referred to as ATF) device which is arranged to position a magnetic head at a recording track formed on a magnetic disc.
2. Description of the Related Art
As a result of recent advancement of the video recording technology, the recording methods employed in various video recording fields have come to attract attention. For example, the electronic still video (SV) camera is arranged to record video signals for fifty frames in a concentric state on a magnetic disc.
Generally, the recorded signal is reproduced from the magnetic disc by shifting a magnetic head to an applicable recording track with a stepping motor. In addition to that, the head is accurately positioned at the recording track by performing an ATF action. The conventional ATF action has been, for example, as shown in a flow chart in FIG. 1(a) of the accompanying drawings. The ATF action is performed to shift the magnetic head to the recording track position in such a way as to bring the reproduction output of the magnetic head to a maximum level thereof by comparing the sampled levels of an RF signal reproduced from the magnetic head (see FIGS. 1(b)-2 and 1(b)-2.
Referring to FIG. 1(a), the envelope level of the reproduced RF signal is sampled at a step 101. At a step 102: A check is made to see if the current sampling is the first sampling. If so, the flow of operation comes to a step 103. At the step 103: the magnetic head is shifted toward the outer circumference side of the magnetic disc to the unit moving extent (i.e., one step) of the stepping motor. The flow comes to a step 106. At the step 106: This envelope level is stored in a memory. The flow then comes back to the step 101. If the current sampling is found to be not the first sampling at the step 102, the flow comes from the step 102 to a step 104. At the step 104: The currently sampled envelope level is compared with a previously sampled envelope level stored in the memory. If the current envelope level is found to be higher than the previous envelope level, the flow proceeds to a step 105. At the step 105: The head is shifted one step further in the direction of the preceding shift. At a step 106: The current envelope level is stored in the memory and the flow comes back to the step 1.
Further, if the current envelope level is found to be lower than the previous envelope level at the step 104, the flow branches off to a step 107. At the step 107: The current head position is checked to see if it is located one step away from the initial position on the outer circumference side. If so, the flow proceeds to a step 108 to have the head shifted two steps toward the inner circumference side by the stepping motor and then comes back to the step 101.
If the head position is found to be on the inner circumference side of the initial position at the step 107, the flow proceeds to a step 109 to have the head shifted by one step in the direction reverse to the preceding shift. The ATF action then comes to an end.
More specifically, the ATF action is described as follows: As shown in FIGS. 1(b)-1 and 1(b)-2, the action is assumed to begin when the head is located in a position "a" which is off the track on the outer circumference side thereof. The envelope level of the reproduced RF signal obtained in this position is stored. Then, this stored level is compared with the envelope level obtained in another position "b" by shifting the head one step toward the outer circumference side by means of the stepping motor. In this instance, the envelope level lowers as the head moves further away from the track. When the envelope level thus lowers, the head is shifted two steps in the direction reverse to the preceding shift, i.e., toward the inner circumference side of the magnetic disc, to bring the head to a position "c". The head is then shifted further stepwise while comparing the envelope level with a previous level at each step. The head is shifted back one step when the envelope level again comes to lower (at a position "e") and the ATF action comes to an end (at a position "f"). The head thus can be correctly positioned on the recording track.
This action accurately shifts the head either toward the outer circumference side or toward the inner circumference side of the magnetic disc with adequate responsivity.
However, since the magnetic head is shifted by the stepping motor generally through a mechanical part such as gears, there arises some hysteresis due to a mechanical play. Besides, the hysteresis varies with the ambient temperature as well as the degree of wear. When the hysteresis increases to a degree corresponding to one step or thereabout, the ATF action becomes as shown in FIGS. 1(c)-1 and 1(c)-2. Referring to FIGS. 1(c)-1 and 1(c)-2, in this case, with the magnetic head assumed to be shifted from the outer circumferential side of the disc and the ATF action to begin from a position "g", the head is first shifted toward the outer circumference side. However, with the hysteresis corresponding to one step, the head is merely shifted to a position "h" which is almost the same as the position "g" and hardly allows comparison. As a result, the ATF action comes to an end at a position "j". Under that condition, it is hardly possible to accurately position the head on the recording track.
Further, in the ATF action, the magnetic head is controlled to be shifted to the position where the maximum (envelope) level of the reproduced RF signal is obtained. However, with a stepping motor used for shifting the head, the motor comes to a stop in a digital position. Theoretically, therefore, the maximum error of the ATF action becomes 1/2 step from the position where the envelope level reaches its maximum value. Then, with the above-stated hysteresis added to the maximum error, the arrangement to simply shift the head by one step in the direction reverse to the preceding shift at the step 109 when the peak of the envelope level is passed during the ATF action brings a maximum possible error between the shifted position of the head and the peak position of the envelope level up to a value of "1/2 step+the degree of error caused by the hysteresis". This error exceeds an allowable limit for the electronic still video camera which is arranged to perform recording and reproducing operation on each recording track measuring 60 .mu.m in width and having a guard band of 40 .mu.m. This has presented a serious problem.